English

• In recent years, the rapid detection of dangerous chemicals has attracted much attention among researchers, such as toxic organic small molecules, nitroaromatic explosive and heavy metal ions, which pose serious threat to human health and the environment. As a new type of crystalline materials, metal-organic coordination polymers (MOCPs) have received extensive attention because of their potential applications in luminescence sensing^[2], gas storage and separation^[3], magnetism^[4], and so on. Among them, fluorescent sensing presents great application prospect owing to its high selectivity, short response time, high sensitivity and operability^[5]. For example, some MOCPs show excellent luminescence sensing for heavy metal ions (Cu²⁺, Pb²⁺, Cr⁶⁺, etc.), anions and organic molecules^[6-8].

  It is generally known that Fe³⁺, CrO₄^2- and Cr₂O₇^2- ions are very important ions. Fe³⁺ plays important role in oxygen metabolism, oxygen absorption and electron transfer, and CrO₄^2- and Cr₂O₇^2- ions have been widely used as oxidants. But an excess intake of Fe³⁺ and Cr⁶⁺ ions can cause cytotoxicity and carcinogenicity to the human body^[9-10]. Therefore, the design of fluorescent materials with selective and sensitive recognition of Fe³⁺, CrO₄^2- and Cr₂O₇^2- ions is very imperative. Meanwhile, many fluorescence MOCPs are unstable in aqueous medium, so it remains a huge challenge to synthesize water-stable fluorescent MOCPs materials.

  Considerable efforts have been devoted to synthesize fluorescence MOCPs with diverse structures and excellent properties, but it is still difficult to design and prepare the ideal MOCPs materials because the self-assembly process of MOCPs depends on many factors. Numerous studies have shown that ligands and metal ions are the decisive factors for successful MOCPs construction^[11]. Based on the above consideration, in this work, 6-(3, 5-dicarboxylphenyl)nicotinic acid (H₃DCPN) was chosen to construct fluorescent MOCPs based on the following advantages: (i) three carboxylate groups of H₃DCPN ligand can completely or partially deprotonate, which adopt multiple coordination modes to construct MOCPs with diverse structures; (ii) the rigid of H₃DCPN ligand is beneficial to enhance the stability of MOCPs. Finally, through the“mixedligand”strategy, two novel MOCPs, [Cd(HDCPN)(1, 4-bib)_0.5(H₂O)₂]_n (1) and {[Zn(HDCPN)(1, 2-bimb)]·H₂O}_n (2) (1, 4-bib=1, 4-bis(1-imidazoly)benzene, 1, 2-bimb=1, 2-bis(imidazol-1-ylmethyl)benzene), have been successfully constructed by the reaction of H₃DCPN and Cd(Ⅱ)/Zn(Ⅱ) ions (Scheme 1). Furthermore, the solid fluorescence of 1 and 2 were detected. And the fluorescence sensing properties of 1 to different metal cation and anion in aqueous solution were further investigated in detail.

  Scheme 1

  

  Scheme 1.  Structures of the organic ligands

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  1.   Experimental

  1.1   Materials and physical measurements

  All the chemicals used in the experiments were commercially available. The IR spectra were recorded on a FTIR-8400s spectrometer. A Vario MACRO cube elemental analyzer was used to measure the contents of C, H and N. The powder X-ray diffraction patterns were confirmed on a Rigaku D/Max-2500 PC diffractometer (Mo Kα, 0.071 073 nm) at 50 kV, 30 mA with a 2θ range of 5°~50°. Thermogravimetric analysis was tested by using METTLER TGA analyzer. The luminescent spectra were collected on a F-4600 fluorescence spectrometer.

  1.2   Syntheses of the complexes

  1.2.1   Synthesis of [Cd(HDCPN)(1, 4-bib)_0.5(H₂O)₂]_n (1)

  A mixture of H₃DCPN (0.007 5 mmol, 2.2 mg), 1, 4-bib (0.007 5 mmol，1.6 mg), Cd(NO₃)2·4H₂O (0.015 mmol, 4.6 mg) and H₂O/Ethanol/DMA (2:1:1, V/V, 1.0 mL) was added into a glass tube and vacuumized, sealed and heated to 80 ℃ for 96 h, and then cooled slowly to room temperature. Colorless block crystals were collected (Yield: 41%, based on Cd). Anal. Calcd. for C₂₀H₁₆CdN₃O₈(%): C, 44.55; H, 2.97; N, 7.80. Found (%): C, 44.51; H, 2.32; N, 7.75. IR (KBr pellet, cm^-1): 3 463 (m), 1 707 (m), 1 628 (m), 1 593 (s), 1 521 (m), 1 454 (m), 1 382 (m), 1 160 (m), 839 (w), 789 (m), 734 (m), 684 (w), 470 (w) (Supporting information, Fig.S1).

  1.2.2   Synthesis of {[Zn(HDCPN) (1, 2-bimb)] ·H₂O}_n (2)

  The synthetic process of complex 2 is similar to that of 1 except for using 1, 2-bimb (0.02 mmol, 4.8 mg) instead of 1, 4-bib. And the colorless crystals were obtained. Yield: 48%. Anal. Calcd. for C₂₈H₂₃ZnN₅O₇ (%): C, 55.45; H, 3.79; N, 11.53. Found(%): C, 55.31; H, 3.83; N, 11.51. IR (KBr, cm^-1): 3 439 (m), 3 119 (m), 2 482 (w), 1 777 (s), 1 593 (s), 1 540 (s), 1 364 (s), 1 349 (s), 1 096 (s), 935 (m), 828 (m), 767 (vs), 621 (s) (Fig.S1).

  1.3   Single crystal X-ray diffraction

  Single-crystal diffraction data of 1 and 2 were collected on a Bruker Apex Ⅱ CCD diffractometer (Mo Kα, 0.071 073 nm) at 296 and 293 K, respectively. The structures were solved by direct methods and refined using the full-matrix least-squares method based on F² by the SHELXL-2015^[12]. Non-hydrogen atoms were refined anisotropically. Hydrogen atoms from carbon atoms were generated by geometrical considerations. The solvent molecules in 1 and 2 are highly disordered and were removed by the SQUEEZE program of PLATON^[13]. The primary crystallographic data, selected bond lengths, bond angles and fractional atomic coordinates are given in Table 1, S1 and S2.

  Table 1

  

  Table 1.  Crystallographic data of complexes 1 and 2

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  Complex	1	2
  Empirical formula	C20H16CdN3O8	C28H23ZnN5O7
  Formula weight	538.76	606.87
  Crystal system	Triclinic	Monoclinic
  Space group	P1	P21/n
  a / nm	0.96962(11)	0.89720(18)
  b / nm	1.02661(12)	3.0330(6)
  c / nm	1.36386(16)	1.0158(2)
  α/(°)	70.423(7)
  β/(°)	85.483(7)	103.804(5)
  γ/(°)	64.491(6)
  V / nm3	1.151(2)	2.6844(9)
  Z	2	4
  Dc/ (g·cm-3)	1.555	1.457
  F(000)	538.0	1 208
  μ(Mo Kα) / mm-1	0.997	0.967
  Reflection collected	14 320	22 528
  2θ range for data collection / (°)	4.668~55.442	4.342~55.21
  Independent reflection (Rint)	5 335(0.031 5)	6 179(0.087 2)
  Data, restraint, parameter	5 335, 1, 290	6 179, 228, 435
  GOF	1.018	0.986
  R1, wR2[I > 2σ(I)]	R1=0.029 0, wR2=0.079 1	R1=0.053 8, wR2=0.116 2
  R1, wR2(all data)	R1=0.035 2, wR2=0.083 1	R1=0.093 3, wR2=0.133 6

  CCDC: 1532247, 1; 1999116, 2.

  2.   Results and discussion

  2.1   Descriptions of crystal structures

  2.1.1   Crystal structure of 1

  Complex 1 crystallizes in the triclinic system, with P1 space group. There are one Cd (Ⅱ) ion, one partly deprotonated H₃DCPN ligand, half of a 1, 4-bib molecule and two coordinated water molecules in the asymmetric unit of 1 (Fig. 1). The Cd1 (Ⅱ) ion is seven-coordinated by six O-atoms (Cd1-O1 0.270 00(25) nm, Cd1-O2 0.226 19(23) nm, Cd1-O5A 0.253 17(18) nm, Cd1-O6A 0.231 14(21) nm, Cd1-O1W 0.231 19(20) nm, Cd1-O2W 0.246 48(18) nm) from two different H₃DCPN ligands and two coordinated water molecules, one nitrogen atom from a 1, 4-bib molecule (Cd1-N2 0.222 36(30) nm), presenting a distorted pentagonal bipyramid geometry. The angles of O-Cd-O and O-Cd-N are from 50.978(77)° to 176.415(70)°.

  Figure 1

  

  Figure 1.  Coordination environment of Cd(Ⅱ) in 1

  Probability of ellipsoid is 30%; Symmetry code: A: x, 1+y,-1+z

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  The carboxylate groups of H₃DCPN in 1 are partially deprotonated and adopt chelating (η²) coordination modes (Scheme 2a). The two carboxylate groups of HDCPN^2- ligand and 1, 4-bib linker connect the adjacent Cd(Ⅱ) ions to form 1D chains (Cd…Cd 1.405 61(17) nm) (Fig. 2a and 2b), which were further extended into a 3D structure by π … π interactions (Cg…Cg: 0.367 13(35) and 0.365 05(28) nm, Fig. 2c and 2d).

  Scheme 2

  

  Scheme 2.  Coordination configurations of the H₃DCPN ligands

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  Figure 2

  

  Figure 2.  (a, b) One-dimensional chain structure of complex 1; (c) 3D framework of 1 viewed along c axis; (d) π…π interaction between molecules of complex 1

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  2.1.2   Crystal structure of 2

  Complex 2 crystallizes in the monoclinic system space group P2₁/n. The asymmetric unit consists of one independent Zn (Ⅱ) ion, one H₃DCPN ligand, a 1, 2-bimb linker, and a lattice water (Fig. 3). The Zn1(Ⅱ) ion presents a distorted {ZnN₂O₄} octahedral geometry, and is bridged by four O atoms (Zn1-O1 0.221 79(23) nm, Zn1-O2A 0.197 72(21) nm, Zn1-O3B 0.207 32(24) nm, Zn1-O4B 0.284 32(22) nm) from three distinct HD-CPN^2- ligands, two N atoms (Zn1-N2 0.197 41(27) nm, Zn1-N5A 0.198 24(27) nm) from two different 1, 2-bimb linkers. The angles of O-Zn-N and N-Zn-N are from 81.603(91)° to 176.984(118)°.

  Figure 3

  

  Figure 3.  Coordination environment of Zn(Ⅱ) in 2

  Probability of ellipsoid is 30%; Symmetry codes: A: 1-x, 1-y, 1-z; B: 1-x, 1-y, 2-z

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  The carboxylate groups of H₃DCPN in 2 are partially deprotonated, and adopt chelating (η²) and bridging (μ₂-η¹:η¹) coordination modes (Scheme 2b). Four carboxylate groups from four HDCPN^2- ligands connect with two Zn(Ⅱ) ions to construct a binuclear [Zn₂(COO)₄] cluster (Fig. 4a) with the distance of Zn…Zn being 0.386 20(9) nm, which is extended by HDCPN^2- ligand and 1, 2-bimb linker to generate a 1D chain (Fig. 4b), and is further expanded to a 3D framework through hydrogen bonds (Table S3) and π…π interactions (Cg …Cg: 0.349 43(180) nm) between HDCPN^2- ligand and 1, 2-bimb linker (Fig. 4c).

  Figure 4

  

  Figure 4.  (a) [Zn₂(COO)₄] cluster of 2; (b) 1D chain structure of complex 2; (c) 3D structure of 2; (d) π…π interaction between molecules of 2

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  2.2   IR analysis, thermal analysis and X-ray powder diffraction analysis

  The IR measurements of 1 and 2 were carried out in a wavenumber range of 4 000~400 cm^-1. The absorption peaks at 1 628 and 1 382 cm^-1 for 1 and 1 593 and 1 364 cm^-1 for 2 are attributed to asymmetrical and symmetrical stretching vibrations of-COOH from HDCPN^2- ligand, respectively. The presence of strong peak at 1 650~1 710 cm^-1 shows that the H₃DCPN ligand is partially deprotonated^[14] (Fig.S1). The thermal stability curves of 1 and 2 are displayed in Fig.S2. For 1, the weight loss of 6.3% (Calcd. 6.7%) below 100 ℃ corresponds to the release of two coordinated water molecules, and the structure began to collapse above 264 ℃. 2 presented the first weight loss of 2.5% (Calcd. 2.9%) under 248 ℃, which is attributed to the release of a lattice water. The structure began to decompose quickly at about 270 ℃. Furthermore, X-ray powder diffraction analyses of 1 and 2 indicate that the experimental diffraction patterns are basically consistent with simulated ones, indicating the good phase purity (Fig.S3).

  2.3   Luminescent properties

  The solid-state fluorescence emission spectra of H₃DCPN, 1 and 2 were investigated at room temperature (Fig. 5). The maximum emission peaks of H₃DCPN, 1 and 2 are at 420 nm (λ_ex=280 nm), 392 nm (λ_ex=280 nm), 387 nm (λ_ex=280 nm), respectively. Compared to H₃DCPN ligand, the obvious blue-shifts of the emission peaks of 1 and 2 may be assigned to the increase of the conformational rigidity of H₃DCPN ligand, and the decrease of non-radioactive decay due to the coordination between Cd(Ⅱ)/Zn(Ⅱ) ions and H₃DCPN^[15].

  Figure 5

  

  Figure 5.  Solid-state fluorescence emissions for H₃DCPN ligand, 1 and 2 at room temperature

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  Because complex 1 remains stable in aqueous solution and shows luminescent properties in the solid state at ambient temperature, the fluorescence sensing properties of 1 to different metal cation and anion were measured in aqueous solution. Ground samples of 1 (3 mg) were immersed in 3 mL M(NO₃)_n aqueous solutions (0.01 mol·L^-1, M=Cd²⁺, Zn²⁺, Ba²⁺, Co²⁺, K⁺, Al³⁺, Cu²⁺, Pb²⁺+, Ni²⁺, Na⁺, Ag⁺ and Fe³⁺). As displayed in Fig. 6, the luminescence intensity of Mⁿ⁺@1 suspensions mainly depends on different cations. It is worth mentioning that Fe³⁺ shows distinct quenching effects with the quenching rates being more than 98%. Meanwhile, the anti-interference experiment was also detected in the presence of other ions (Fig. S4). The results revealed that the fluorescence intensity of 1 was changed slightly in the absence of Fe³⁺, and the fluorescence showed obviously quenched behavior after adding Fe³⁺ into the above solutions.

  Figure 6

  

  Figure 6.  Photoluminescence intensities of 1 dispersed in M(NO₃)_n aqueous solutions

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  The luminescent sensing sensitizations were further examined by changing Fe³⁺ concentration, titration experiments revealed that the luminescent intensities of 1 were gradually decreased as the concentration of Fe³⁺ increased. When the concentration of Fe³⁺ reached 0.80 mmol·L^-1, the luminescence intensity of 1 was obviously quenched. The nonlinear curve fitting between the concentrations of Fe³⁺ and luminescence intensity (I₀/I) can be followed the exponential equations: I₀/I=0.177 1exp(c_M/0.161 7)+1.023 4, where I₀ is the fluorescence intensity of 1@H₂O and I is the luminescent intensity of 1@Fe³⁺ suspensions, c_M is the concentration of Fe³⁺ (mmol·L^-1) (Fig. 7). For Fe³⁺, in the low concentration range, the plot of I₀/I vs c_M is nearly linear and can be fitted by the Stern-Volmer (SV) equation I₀/I=1+K_SVc_M. The fluorescence quenching constant (K_SV) was calculated to be 4.74×10⁴ L·mol^-1. The detection limit (3σ/K_SV) of 1 for Fe³⁺ was calculated to be 1.32×10^-4 mol·L^-1 (σ is the standard deviations for five repetitive fluorescence experiment of blank solutions), which is similar to the reported values of other luminescent materials in the literature^[16-17]. And the low detection limit demonstrates that 1 shows highly sensitive detection toward Fe³⁺.

  Figure 7

  

  Figure 7.  Luminescent emission spectra of 1 dispersed in aqueous solution upon incremental addition of Fe³⁺

  Inset: plot of luminescence intensity (I₀/I) vs Fe³⁺ concentration

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  To further investigate the luminescence sensing of 1 for different anions, 3 mg samples were dispersed in 3 mL K_mX aqueous solution (0.01 mol·L^-1, X=Cl^-, HCO₃^-, I^-, SO₄^2-, Br^-, SCN^-, H₂PO₄^-, CO₃^2-, HPO₄^2-, CrO₄^2-, and Cr₂O₇^2-). As shown in Fig. 8, the luminescence of Cr₂O₇^2- and CrO₄^2- anions for 1@H₂O suspension was almost completely quenched, and the quenching rate was as high as 98.2% and 97.1%, respectively.

  Figure 8

  

  Figure 8.  Photoluminescence intensities of complex 1 dispersed in the solutions containing different anions

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  As shown in Fig. 9 and 10, when the concentrations of Cr₂O₇^2- and CrO₄^2- were 0.6 and 0.7 mmol·L^-1, respectively, the fluorescence intensity of 1 was almost completely quenched. The curves of the luminescence intensity and the concentrations of Cr₂O₇^2- and CrO₄^2- were fitted by the exponential equations: I₀/I=0.178 6exp (c_Cr2O2^7-/0.148 1)-0.654 1 and I₀/I=2.919 1exp(c_CrO4^2-/ 0.356 0)-1.768 3, respectively (Fig. 9 and 10, Inset).

  Figure 9

  

  Figure 9.  Luminescent emission spectra of 1 dispersed in aqueous solution upon incremental addition of Cr₂O₇^2-

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  Figure 10

  

  Figure 10.  Luminescent emission spectra of 1 dispersed in aqueous solution upon incremental addition of CrO₄^2-

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  At low concentrations, the SV plots for Cr₂O₇^2- and CrO₄^2- were approximately linear and the K_SV values of Cr₂O₇^2- and CrO₄^2- were calculated to be 2.194×10³ and 1.096×10³ L·mol^-1. The detection limit for Cr₂O₇^2- and CrO₄^2- could reach 1.98×10^-3 and 4.38×10^-3 mol·L^-1. And the structure of 1 immersed in aqueous solutions containing Fe³⁺, Cr₂O₇^2- and CrO₄^2- ions is stable, which can be confirmed by the PXRD patterns of the samples before and after fluorescence experiments (Fig. S5). Moreover, three cycles of the Fe³⁺/Cr₂O₇^2-/CrO₄^2- sensing experiments indicated that 1 showed good cyclability (Fig.S6~S8). These results indicate that 1 is expected to be used as a potential fluorescent material for detecting Cr₂O₇^2- and CrO₄^2- in aqueous system^[18-19].

  The quenching mechanism of Fe³⁺/Cr₂O₇^2-/CrO₄^2- for 1 can be ascribed to the competitive adsorption between the framework of 1 and metal ions, which can be confirmed by the UV-Vis absorption spectra of 1, Fe³⁺, Cr₂O₇^2- and CrO₄^2-, since there were partial overlaps between the excitation spectra of 1 and the absorption spectra of Fe³⁺/Cr₂O₇^2-/CrO₄^2- (Fig.S9). Besides, the electron-transfer and weak interaction between the analyte and Fe³⁺/Cr₂O₇^2-/CrO₄^2- ions also play a part role^[20-22].

  3.   Conclusions

  In summary, two fluorescent MOCPs have been prepared based on 6-(3, 5-dicarboxylphenyl)nicotinic acid ligand and Cd(Ⅱ)/Zn(Ⅱ) in the presence of imidazole ligands, and their structures are one-dimensional chain. Complexes 1 and 2 present good stability. In particular, 1 shows sensitive and selective sensing for Fe³⁺, Cr₂O₇^2- and CrO₄^2- in aqueous solutions, which means that it can be employed as a chemical sensor in detecting Fe³⁺, Cr₂O₇^2- and CrO₄^2-.

  Supporting information is available at http://www.wjhxxb.cn

  
  Acknowledgments: The authors gratefully acknowledge the financial support of this work by the fund for Natural Science Foundation for Young Scientists of Shanxi Province (Grant No. 201901D211443), the fund for Shanxi“1331 Project”Key Innovative Research Team (Grant No. PY201817), the Jinzhong University“1331 Project”Key Innovation Team (Grant No.jzxy-cxtd2019005) and Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (STIP) (No. 2020L0590).
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• 

  Scheme 1  Structures of the organic ligands

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  Figure 1  Coordination environment of Cd(Ⅱ) in 1

  Probability of ellipsoid is 30%; Symmetry code: A: x, 1+y,-1+z

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  Scheme 2  Coordination configurations of the H₃DCPN ligands

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  Figure 2  (a, b) One-dimensional chain structure of complex 1; (c) 3D framework of 1 viewed along c axis; (d) π…π interaction between molecules of complex 1

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  Figure 3  Coordination environment of Zn(Ⅱ) in 2

  Probability of ellipsoid is 30%; Symmetry codes: A: 1-x, 1-y, 1-z; B: 1-x, 1-y, 2-z

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  Figure 4  (a) [Zn₂(COO)₄] cluster of 2; (b) 1D chain structure of complex 2; (c) 3D structure of 2; (d) π…π interaction between molecules of 2

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  Figure 5  Solid-state fluorescence emissions for H₃DCPN ligand, 1 and 2 at room temperature

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  Figure 6  Photoluminescence intensities of 1 dispersed in M(NO₃)_n aqueous solutions

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  Figure 7  Luminescent emission spectra of 1 dispersed in aqueous solution upon incremental addition of Fe³⁺

  Inset: plot of luminescence intensity (I₀/I) vs Fe³⁺ concentration

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  Figure 8  Photoluminescence intensities of complex 1 dispersed in the solutions containing different anions

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  Figure 9  Luminescent emission spectra of 1 dispersed in aqueous solution upon incremental addition of Cr₂O₇^2-

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  Figure 10  Luminescent emission spectra of 1 dispersed in aqueous solution upon incremental addition of CrO₄^2-

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  Table 1.  Crystallographic data of complexes 1 and 2

  Complex	1	2
  Empirical formula	C20H16CdN3O8	C28H23ZnN5O7
  Formula weight	538.76	606.87
  Crystal system	Triclinic	Monoclinic
  Space group	P1	P21/n
  a / nm	0.96962(11)	0.89720(18)
  b / nm	1.02661(12)	3.0330(6)
  c / nm	1.36386(16)	1.0158(2)
  α/(°)	70.423(7)
  β/(°)	85.483(7)	103.804(5)
  γ/(°)	64.491(6)
  V / nm3	1.151(2)	2.6844(9)
  Z	2	4
  Dc/ (g·cm-3)	1.555	1.457
  F(000)	538.0	1 208
  μ(Mo Kα) / mm-1	0.997	0.967
  Reflection collected	14 320	22 528
  2θ range for data collection / (°)	4.668~55.442	4.342~55.21
  Independent reflection (Rint)	5 335(0.031 5)	6 179(0.087 2)
  Data, restraint, parameter	5 335, 1, 290	6 179, 228, 435
  GOF	1.018	0.986
  R1, wR2[I > 2σ(I)]	R1=0.029 0, wR2=0.079 1	R1=0.053 8, wR2=0.116 2
  R1, wR2(all data)	R1=0.035 2, wR2=0.083 1	R1=0.093 3, wR2=0.133 6

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